Compared to traditional white thermal lamps, cold cathode fluorescent lamps (CCFL) have many advantages such as higher efficiency and longer service life. Therefore, an important number of liquid crystal display (LCD) presently uses CCFL as light source. To achieve a stable operation of the CCFL, the power frequency needed is about 30 KHz through 80 KHz without the stringed wave from the DC current part while the operating voltage is approximately constant. The illumination of the lamp is determined according to the tube current there through. The voltage needed to turn on the lamp is higher than the normal stable operating voltage 2 to 2.5 times. The turn on voltage and operating voltage of the CCFL are determined from the size of the CCFL. Traditional 14″, 15″ LCD screens incorporate CCFL that require a turn-on voltage of about 1400 Vrms, and an operating voltage of about 650 Vrms at the highest normal current of 7 mA. To regulate the CCFL, a common control method is the use of an electrical stabilizer such as a typical fixed frequency operation full bridge phase shift converter that can convert direct current to alternating current.
As shown in FIG. 1, a typical fixed frequency operation full bridge phase shift converter comprises a resonance inductor 105, a capacitor 106 circuit, a lamp 107, NMOS switches 101, 102, 103, 104, and DC current 108. The turn-on or turn-off of NMOS switches 101, 102, 103, 104 are respectively controlled via the gale voltages VG1, VG2, VG3, VG4. Typical control signals are four similar constant frequencies with a same duty cycle slightly smaller than 50% and different square waves. A typical fixed frequency operation full bridge phase shift converter moves the phase of the control voltage so as to use different sizes of phase retardations to generate different output powers. FIG. 2A and FIG. 2B schematically illustrate the operation time/sequence of a typical fixed frequency operation full bridge phase shift converter. To prevent the NMOS switches 101, 102 (and 103, 104), directly connected to each other, to be simultaneously turned off, which causes a power loss, the control signals VG1, VG2 (and VG3, VG4) must maintain a phase retardation of 180°. The phase retardation of VG1, VG3 in FIG. 2A is smaller than that of FIG. 2B, which generates a higher duty cycle VAB and more power outputs.
However, the present use of traditional fixed frequency operation full bridge phase shift converter in LCD screens presents several problems. Within present LCD devices, the DC voltage provided by the circuit is only about 10 to 20 volts. The electrical stabilizer of the CCFL of FIG. 1 needs a direct voltage of hundreds of volts to operate. Moreover, this stabilizer uses a NMOS as power switch. As a result, when it is driven, the voltage VG1 (respectively VG2) at the point A (respectively B) must be cautiously increased. Additional step-up circuits thus must be included within the driving circuits of the NMOSFET power switches 101, 104.